1. Field of the Invention
The present invention relates to a MUT (Micromachined Ultrasound Transducer).
2. Description of the Related Art
An ultrasound diagnosis method is commonly employed in which ultrasound is transmitted to walls of a body cavity and the state of the body cavity is converted into images on the basis of the echo signals thereof for the purpose of making diagnoses. An ultrasound endoscope is one of the devices used for this ultrasound diagnosis method.
The ultrasound endoscope has an ultrasound probe at the distal end of an insertion tube to be inserted into body cavities, and this ultrasound probe converts electric signals into ultrasound in order to transmit the ultrasound to body cavities and also receives ultrasound reflected in the body cavities in order to convert the received ultrasound into electric signals.
Conventionally, ceramic piezoelectric material PZT (lead zirconate titanate) has been used for piezoelectric elements that convert electric signals into ultrasound. However, micromachined ultrasound transducers (hereinafter, referred to as MUTs) have attracted attention. MUTs are among a category of devices that are referred to as micro electro-mechanical systems (MEMS).
Also, in recent years, capsule endoscopes have been realized that are swallowed into body cavities in order to obtain images of the body cavities (for example, Patent Document 8). By using this ultrasound diagnosis medical capsule endoscope, it is possible to make ultrasound diagnoses at sites at which it was previously difficult to make diagnoses.
Because the conventional MUTs have been configured to have membranes of respective transducer cells that are continuous along the plane direction, the continuous configuration has interfered with the deformation of the membranes of the respective transducer cells and the transverse waves have leaked to the adjacent transducer cells, and thereby the efficiency of the electromechanical conversion has been reduced.
Also, the membranes have been supported by the peripheral portions, and accordingly the bending vibrations of the membranes have leaked to the supporting members as longitudinal vibrations so that the mechanical quality factor Q has been reduced and the amplitude of the membrane has been small. Thereby, the S/N ratio has been reduced so that ultrasound diagnosis images have been deteriorated. Also, the conventional MUTs, especially cMUTs (capacitive micromachined ultrasound transducers) have required DC bias for both the reception and the transmission, and accordingly a driving control method that uses as small an amount of DC bias voltage as possible has been important. This DC bias has had to be applied for far longer periods of time for the reception than for the transmission, and thus a configuration that realizes reception without DC bias has been desired.
Also, the techniques relating to the present invention are disclosed in, for example, patent documents 1 through 8 below.
Patent Document 1:
Japanese Patent Application Publication No. H07-274287
Patent Document 2:
Japanese Patent Application Publication No. H08-274573
Patent Document 3:
Japanese Patent Application Publication No. 2004-274756
Patent Document 4:
U.S. Pat. No. 6,262,946
Patent Document 5:
U.S. Pat. No. 6,328,696
Patent Document 6:
U.S. Pat. No. 6,328,697
Patent Document 7:
Publication in Japan of translation of PCT International
Patent Application No. 2004-503313 (WO 2001/097562)
Patent Document 8:
Japanese Patent Application Publication No. 2004-350705
Non-Patent Document 1:
Pages 149 through 152 in the Twelfth version of Volume 1 of “Onkyo Kogaku Genron” written by Tsuyoshi Itoh and published by CORONA PUBLISHING CO., LTD, in Dec. 10, 1980